IB Chemistry - Stoichiometry

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Due to the incredibly small size of the basic particles of matter there are fantastic numbers of them in the masses used in everyday life. Amedeo Avogadro is the scientist most credited with work done in this domain.

Syllabus reference

Structure 1.4.1 - The mole (mol) is the SI unit of amount of substance. One mole contains exactly the number of elementary entities given by the Avogadro constant.

  • Convert the amount of substance, n, to the number of specified elementary entities.


  • An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or a specified group of particles.
  • The Avogadro constant NA is given in the data booklet. It has the units mol–1.

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Avogadro's number

Avogadro defined the number of particles of a substance that have a mass equivalent to the relative mass of that substance. He found that the number of atoms in 12 g of carbon was equal to approximately 6.02 x 1023

This number was given his name - the Avogadro constant.

Avogadro's number = 6.02 x 1023

An Avogadro number of particles = 6.02 x 1023 particles

The Avogadro number or constant may be represented by the letter 'L' and when dealing with this number of particles we say that there is an 'amount' of 1 mole.

1 mole of any substance contains an Avogadro number of particles of that substance = 6.02 x 1023 particles


1 mole of iron contains 6.02 x 1023 iron atoms

1 mole of chlorine gas contains 6.02 x 1023 chlorine molecules

1 mole of electrons contains 6.02 x 1023 electrons (also called a Faraday of charge)

Note that the Avogadro constant simply describes a specific number of any particle, be they atoms, molecules, electrons or even armchairs!

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